The invention relates to a forged hollow axle and a method for making the same.
WO 2004/108234 A1 discloses a forged hollow axle which is manufactured from two half shells prepared by press forging and joint along the longitudinal main extension of the axle. By modifying the cross sectional shape and/or by introducing ribs in the axle body torsional stiffness and bending strength can be improved compared to axles with massive axle bodies. For instance, in areas where a high torsional stiffness or resistance is desired, such as the swan neck areas of the axle, a more marked u-shape is generated in the half shells. The swan necks are arranged at the free ends of the front axle body which swan necks connect a pair of king pin supports to the central portion of the axle body. In other areas where great bending stiffness or resistance is desirable, transverse ribs are retained between the opposite vertical sides of the axle profile.
In U.S. Pat. No. 2,685,479 A a tubular axle beam is disclosed which is made of two forged preformed halves which are joined by a longitudinal weld.
It is desirable to provide a forged hollow axle with improved stiffness characteristics, particularly by providing a controlled relationship of bending stiffness and torsional stiffness of the axle.
It is also desirable to provide a method for making a forged hollow axle with improved stiffness characteristics.
A forged hollow axle is proposed, particularly a steerable axle, designed for a commercial vehicle, the axle having a main longitudinal extension in y-direction, a width in x-direction and a height in z-direction and being composed of at least two half shells fixedly joined at an interface which extends along its longitudinal extension. A material thickness d in one or both of the half shells is distributed in the main longitudinal extension and in one or both of width and height to provide a predetermined relation of a torsional stiffness and a bending stiffness in one or more spatial directions in one or more axle sections.
The invention allows, according to an aspect thereof, for a control stiffness distribution along the axle by distributing the material of the half shells in a desired way. The material can be distributed, by way of example, at least in two dimensions, particularly in all three dimensions. In a commercial vehicle, good handling properties of the vehicle, e.g. for bump travel and wind-up brake motion of the axle, and its front suspension installations are dependent on the suspension component stiffness such as the resistance of the leaf spring against, wind up (“wind-up stiffness”) and reaction bushings and the like. A front axle itself affects it any ways the handling of the vehicle with its stiffness between the axle ends. The invention allows for good handling, properties both for air suspension and leaf spring suspension systems, particularly at front suspension systems. The known forged axles used today are optimized for strength and weight aspects instead and provide no means to create a stiffness variation which is favourable to optimize the handling of the vehicle except when increasing, weight and costs strongly.
Favourably, the torsional stiffness can be a predetermined percentage higher than the bending stiffness in the x-direction between the spring mounting positions on the vehicle, and/or the bending stiffness in z-direction can be a predetermined percentage higher than the bending stiffness in x-direction outside the spring positions, and the torsional stiffness can be a predetermined percentage higher than the bending stiffness in the x-direction outside the spring positions.
The material distribution in the half shells is generated by die forging. Particularly, hot die forging can be used.
Favourably, the material of the axle body can be distributed in at least two, particularly in all three dimensions, i.e. it is possible to generate thickness variations in x-, y-, and z-direction. A desired amount of material and material thickness can be generated at desired locations in the half shell. Die forging, in particular hot die forging, allows for a complete reshaping of the material in the axle, thus improving its material properties. Other than press forging or bending of corrugated material which already has some thickness variations prepared in the blank material, the die forged material has properties different from materials prepared by the other methods. In particular it can be observed in cross sectional cuts that the die forged material shows a multitude of fibre like structures which are created by alloyed material concentration variations generated during die forging. This causes an anisotropic material strength both depending on the direction with respect to the fibre and on the local density of the fibres in the fibre like structures. Areas with a higher density give rise to a higher strength even if the area is subject to stress concentration due to a radius such as a corner. A higher strength of the half shell and accordingly of the axle follows from this effect, thus allowing to prepare half shells with favourably thin walls along the z-direction. The stiffness of the half shell can be increased by moving as much material as possible out of relevant zones of the half shells which also yields an advantageous weight reduction of the half shell and the axle composed of such half shells.
According, to a favourable embodiment, the material distribution has an average thickness along an extension in z-direction which is lower than an average thickness along an extension in x-direction, which results in a favourable weight reduction as well as favourable stiffness relations concerning bending stiffness and torsional stiffness. Bending stiffness means a resistance the axle has against bending it about an axis in the x-direction or z-direction. Torsional stiffness (also called rotation stiffness) means a resistance the axle has against a rotation about an axis in y-direction.
According to a favourable embodiment, the material may be distributed so as to yield the torsional stiffness to be larger than the bending stiffness in x-direction in a middle section. Favourably, the axle provides better handling properties when driving over a bump or at wind-up when braking. The general advantage is that both the torsional stiffness and the bending stiffness around the z-axis get, much higher with a tubular cross section even with a reduction of material volume and weight. A relation between the stiffnesses are the one found from an appropriate analysed model and is typically similar even after further optimisation loops for a final product.
According to a favourable embodiment, the material distribution may be established so as to yield the bending stiffness in z-direction to be similar with the bending stiffness in x-direction in a side section outside the middle section.
Favourably, the axle provides better handling properties when driving over a bump or at wind-up when braking. An advantage is that the steering geometry can be adapted for the suspension characteristics with minimum of effects from the stiffness of the axle.
According to a favourable embodiment, the half shells may be joined by flash-butt welding. Flash-butt welding avoids substantial damage to the material of the half shells. The joint which is achieved with this technique is essentially as strong as the parent material of the half shells without the aid of any filler material. Impurities and oxides are pushed out of the weld zone so that the inner weld material is sound and free of oxides or cast material.
According to a favourable embodiment, the torsional stiffness and the bending, stiffness may differ at least by a factor of 1.5, particularly at least by a factor of 2. A good roll steer effect can be achieved. Important stiffness characteristics can be increased locally for a robust suspension design, favourably for a robust front suspension design.
According to a favourable embodiment, the middle section may be arranged between spring positions of the axle and/or the side section may be a swan neck section of the axle. Favourable steering and handling properties can be achieved for the axle.
According, to a favourable embodiment, the axle may have a design of a steerable axle comprising king pin receptacles at its free ends, and by a design of a non-drivable axle or an axle drivable by individual wheel drives.
According to a favourable embodiment, the axle may have a design of a front axle. Front steering handling can be improved both for bump handling and brake wind-up handling.
Further, a method is proposed for manufacturing a forged hollow axle, particularly a steerable axle, characterized by the steps of preparing a first half shell and a second half shell by die forging to establish a material distribution which yields a material thickness in one or both half shells to provide a predetermined relation of a torsional stiffness and a bending stiffness in one or more spatial directions in one or more axle sections; and                joining the half shells by flash-butt welding for maintaining the material properties of the half shells after welding.        
The half shells can be manufactured with well defined properties which result in superior stiffness properties and reduced weight. The joining technique of the half shells can be performed automated and computer controlled which results in a high and reliable output of axles, particularly steerable front axles tier both leaf spring and air spring systems. The material properties of the half shells and/or the axle can be derived form model calculation as is usual in the art.
Further, a vehicle comprising a forged steerable hollow axle is proposed. The vehicle provides good handling properties due to the improved stiffness of the axle, particularly the steerable front axle.
Generally, the invention allows an advantageous increase of the torsional stiffness (rotation about the y-direction) with e.g. 200% at the axles swan neck section, compared to massive forged axles. As a result, a steering error at both bump and brake handling can be reduced as well as a robust steering behaviour be achieved which is independent of the axle's swan neck height. Thus, the needed steering geometry variation can be simplified and the number of components at large variations on axle geometry and installation heights can be reduced.